Corroles are tetrapyrrole macrocycles that are closely related to porphyrins, with one carbon atom less in the outer periphery and one NH proton more in their inner core. They may also be considered as the aromatic version (identical skeleton) of the only partially conjugated corrin, the cobalt-coordinating ligand in Vitamin B12.
Porphyrins, phthalocyanines, and related macrocycles are extensively investigated in many applications, among them photodynamic therapy and catalysis. The most promising candidates for selective association to tumor cells are amphiphilic derivatives and chiral metal complexes are of prime importance for the utilization in asymmetric catalysis. Both structural types present a significant synthetic challenge because of the high symmetry of the most common precursors, tetraarylporphyrins and phthalocyanine. On the other hand, the less symmetric corroles could be very useful candidates for the above mentioned purposes. However, this potential was not explored until most recently because of the non-availability of simple synthetic methodologies for the preparation of corroles.
For decades, the chemistry of corroles was almost entirely limited to derivatives with fully alkylated β-pyrrole positions,1 with only three examples of meso-only substituted corroles.2 This situation changed dramatically recently with the disclosure by the present inventors (see WO 01/18771) of the first facile methodologies for the synthesis of 5,10,15-triarylcorroles from simple aldehydes and pyrrole:3 about 80 new corroles that are substituted only at the three meso-carbon atoms were reported by now.4 This development finally opened the gate for extensive investigations of corroles in the many applications that tetraarylporphyrins are constantly utilized.5 Particularly, the metal complexes of 5,10,15-tris(pentafluorophenyl)corrole (1 in Scheme 1) were shown to be very efficient catalysts for atom (oxygen) and group (carbene, nitrene) transfer to organic substrates.6 In fact, for the latter reactions the corrole metal complexes are significantly more efficient than analogous porphyrins.6c, 6e In addition, a water-soluble derivative of 1 (obtained by replacing its para-F atoms by pyridylium cations) was shown to be more efficient in inhibiting growth factors in tumor cells than analogous porphyrins and quite novel photophysical properties of non-transition metal corroles were recently disclosed.7,8 
The two major structural peculiarities of corroles relative to porphyrins are the presence of three rather than two NH protons in the coordination core and the lower symmetry. Large emphasis was given to the first feature, particularly for stabilization of metal ions in high oxidation states,9 while the other one was quite ignored. For example, although N-substituted corroles were reported as early as 1965,10 the fact that these molecules are chiral was not appreciated until most recently.11 A different aspect is the possibility of selective substitution of the macrocycle's protons, which could not be explored for the traditional corroles because they were fully alkylated at the β-pyrrole carbon atoms.12 On the other hand, electrophilic substitution of porphyrins and phthalocyanines either proceeds to completion or provides an almost intractable mixture of products and isomers.13 For example, even a bis-sulfonated phthalocyanine that was separated from mono- and multi-sulfonated products was shown to be a mixture of at least eight isomers.14 In principle, the situation for meso-only substituted corroles could be better if the four different β-pyrrole carbon atoms display highly significant different reactivities. Otherwise, the number of possible products will be exceedingly large, up to 140 (see Scheme 1 herein).